1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints on a print medium being conveyed, and in particular, to an ink jet printing apparatus that deals with a possible variation in the conveyance amount of the print medium caused during conveyance.
2. Description of the Related Art
Ink jet printing apparatuses form images by fixing droplets of ink or the like to the surface of a print medium as a coloring material. There has recently been a growing demand for improvement of the quality of images printed by the ink jet printing apparatus. In connection with the demand, the volume of each droplet ejected by the ink jet printing apparatus has tended to decrease.
The reduced volume of the ejected droplet decreases the diameter of a print dot after impact onto the print medium. Although depending on the bleeding characteristics of the print medium, the diameter of each dot is about 30 μm on print media corresponding to photographic quality. For dots with a smaller diameter, the demand for the accuracy of the impact position on the print medium is higher. If dots of diameter about 30 μm are formed, ejected droplets need to exhibit, as a tolerable error, an impact accuracy of about 15 μm, corresponding to the range of data (the amount between the maximum value and minimum value of the data), in general, though the accuracy may vary depending on peripheral environments and the type of the apparatus.
For the ink jet printing apparatuses of a serial-scan type that print a print medium by allowing a print head to scan the print medium being conveyed, there has been a growing demand for improvement of the impact accuracy of ejected droplets in both a conveyance direction and a direction intersecting with the conveyance direction. Here, the conveyance direction refers to the direction in which the print medium is conveyed. In general, to be conveyed, the print medium is sandwiched between a pair of rollers at least one of which is rotationally driven. The print medium is then conveyed in the conveyance direction.
However, in the configuration in which the print medium is conveyed in the conveyance direction by the rollers, the conveyance amount of the print medium in the conveyance direction may vary. In particular, when the upstream end of the print medium is released from the rollers, the conveyance amount of the print medium may vary. That is, when the print medium is released from the rollers, the amount by which the print medium moves in the conveyance direction may vary. Thus, an image on the printed print medium may include an unprinted area or a particular part of the image may be scanned an unnecessarily large number of times for printing. When a particular part of the image is scanned an unnecessarily large number of times, a black-stripe like-line may be created in that part. This may degrade the quality of images obtained by the printing.
To solve these problems, Japanese Patent Laid-Open No. 2006-130789 proposes an ink jet printing apparatus that reduces the number of ejection ports through which droplets are ejected when a print medium is released from rollers. This reduces the conveyance amount during each scan and thus a possible error in conveyance amount. The reduced conveyance amount during each scan allows the number of scans to be increased. Moreover, when the print medium is released from the rollers, a gear transmitting rotation to the rollers may backlash to excessively convey and thus misalign the print medium. The misalignment of the print medium is absorbed by shifting the range of active ejection ports in the print head to the upstream side.
Variation in the conveyance amount of the print medium can be reduced by using the printing apparatus disclosed in Japanese Patent Laid-Open No. 2006-130789, for printing. However, the ink impact accuracy may still be varied by an error in conveyance amount. Furthermore, Japanese Patent Laid-Open No. 2006-130789 describes the absorption of an error resulting from the misalignment of the print medium in the direction in which the print medium is excessively conveyed by the backlash of the gear transmitting rotation to the rollers. However, the misalignment may occur in the opposite direction. Furthermore, when a print head ejecting smaller droplets in order to further improve the image quality is used for printing, the print dot diameter is also reduced to increase the required impact accuracy. Therefore, a method has been required which further reduces the error in the conveyance of the print medium.